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http://Keccak.noekeon.org/Keccak-main-2.1.pdf
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http://eprint.iacr.org/
4. Bernstein, D.J.: Second Preimages for 6 (7 (8??)) Rounds of Keccak. NIST mailing list (2010)
5. Duan, M., Lai, X.J.: Improved zero-sum distinguisher for full round keccak-f permutation. Chin. Sci. Bull. 57(6), 694–697 (2012)